Electron tube apparatus



May 28, 1945- G. s. wAcHTMAN 2,400,895

ELECTRON TUBKE APPARATUS Filed oci. 5, 1943 s sheets-sheet 1 j 13A, m.

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y E :En-.1E BY May281946- G. s. WACHTMAN Y 2,400,895

ELECTRON TUBE APPARATUS Fild oct. 5,'1943 s sheets-sheet 3` I N V ENTOR.

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Patented May 28, 1946 andere UNITED STATES PATENT OFFICE ELEoTRoN TUBEAPPARATUS George S. Wachtman, Harrisburg, Pa. Y Application october 5,-1943, serial No. 505,048

8 Claims.

My invention relates broadly to circuit arrangements for electron tubeapparatus and more particularly to circuits for controlling the resonantfrequency of tunable circuits over a variable frequency range.

One `of the objects of my invention is to provide an improved circuitarrangement for controlling the resonant frequency of electricallytunable circuits over a predetermined frequency range by coacting broadand fine frequency control means.

Another object of my invention is to provide a compensating controlsystem for tunable circuits by which broad and fine frequency adjustingmeans operate integrally in the control of the resonant frequency of thecircuit for securing substantially linear frequency changes over arelatively Wide band spread.

Still another object of my invention is to provide a circuit arrangementfor applying the frequency compensating control circuit of my inventionto production test circuits in the manufacture of piezo electric crystalfrequency control elements.

A further object of my invention is to provide a circuit arrangement forcomparing the frequency of rapid production piezo electric crystals witha frequency standard for producing crystals of selected frequency on aquantity basis with minimum expenditure of time.

Other and further objects of my invention reside in improved electriccircuit systems for testing piezo electric crystals as set forth morefully in the specification hereinafter following, in which:

Figure 1 schematically illustrates a resonant circuit embodying thebroad and fine tuning control arrangement of my invention; Fig. 2 showsa modified arrangement of tuning control system embodying my invention;Fig. 3 schematically shows the application of the circuit of Fig. 2 to asystem for testing piezo electric crystals in accordance with myinvention; Fig. 4 shows a further form of frequency control circuitembodying my invention and illustrating the application of low frequencycompensation means in the cirf cuit; Fig. 5 shows the circuitarrangement of my invention with high frequency compensation means addedto the circuit of my invention; Fig. 6 illustrates a furthermodification of the frequency control circuit having both high and lowfrequency compensation means added to the frequency control circuit;Fig. '7 is a curve diagram showing the ideal characteristic curves ofthe circuit arrangements of my invention; Fig. 8 illustrates practicalconditions which may be encountered unless high precision operation issecured in the frequency control system of my invention; Fig. 9 showsthe characteristic curves obtainable upon adjustment of the highfrequency compensation means illustrated in the circuit arrangement ofFig. 5, all other components remaining the same as in Fig. 2; Fig. l0illustrates the characteristic curves obtainable when the low frequencycompensation means is employed in the circuit arrangement shown in Fig.4, all other circuit components remaining the same as in Fig. 2; 11illustrates the characteristic curves obtainable in the circuit of Fig.6 embodying both high and low frequency compensation, all othercomponents remaining the same as in Fig. 2; Fig. l2 is a graph diagramexplaining the theory of operation of the circuit of my invention as inFig. 3; Fig. 13 shows a further modified form of precision adjustmentcircuit embodying my invention; Fig. 14 illustrates another modifiedform of circuit embodying my invention; Fig. 15 shows still anothermodified form of circuit embodying my invention; Fig. 16 is a circuitdiagram of Aa further modification of the circuit of my invention; andFig. 17 shows a further modified circuit embodying my invention in whichthe maximum frequency to `which the system is tunable may be adjustedwith precision,

My invention provides means for controlling the resonant frequency ofelectrically tuned circuits over a predetermined frequency range by abroad tuning control and over a smaller but constant frequency range bya fine tuning control at any setting of the broad tuning control. Myinvention is applicable to many dierent kinds of electrical systems andis illustrated in this specification in conjunction with an electrondischarge device and an indicating meter for testing piezo electriccrystal elements for activity and frequency.

Figure 1 illustrates in the simplest schematic form a resonant circuitembodying such control elements, being the broad tuning control, 2 beingthe line tuning control, and 3 the associated impedance or impedancesnecessary to resonate the circuit. 2 of Fig. 1 is a split-stator ordual-section variable condenser, or two or more variable condensersganged together and controlled from one control. By ganged I mean Y thatthe condensers are arranged so that their trol I, and the impedance 3,and the other section or' ganged Condensers is connected effectively inparallel with the main control I and the impedance 3. The seriallyconnected section of 2 is chosen so that it has as much control of theresonant frequency at maximum capacity setting of I as the parallellyconnected section has at the minimum capacity setting of I, theresultant action being that 2 exerts a practically constant control ofthe frequency at any setting between and including minimum and maximumcapacity of I with any one impedance or lumped impedances.

Fig. 2 illustrates a practical control circuit, the action of which isbasically the same as that of Fig. l. I is the broad tuning condenserand 2 is the iine tuning condenser, 3 being the impedance. 5 and I aretrimmer Condensers, whereby the control action of the fine tuningcontrol 2 may be adjusted, and 8 is a condenser whereby the minimum orlowest tunable frequency of the combination may be adjusted.

Fig. 3 illustrates one use of the circuit of Fig. 2 with an electrondischarge device I8 as a generator or oscillator connected through acommon impedance or reactance 24 to another electron discharge device2|, used in conjunction with a meter 23 as an indicating device toindicate the point or .points of resonance and maximum activity ofeither a standard piezo-element or a piezo-element which it is desiredto test. The resonant circuit elements in Figs. 2 and 3 Vare representedby similar reference characters.

Electron discharge device I8 includes cathode II, control grid I2 andanode I4 with provision for heating cathode II from filament heater I5.High potential source I9 connects to anode I4 as shown. Condenser 20connects between the input and output circuits of the oscillatorconstituted by electron discharge device I8, as illustrated. Electrondischarge device 2l includes cathode 26 and anode 21 connected in aseries path through meter 23 across the circuit through jacks 28 intowhich the standard piezo electric' crystal 25 may be plugged and alsoacross the test jig 29. Test jig 29 has a fixed lplate 30 serv-l ing asa support for the crystal 32 to be tested and a movable plate 3| adaptedto be placed in contact with the upper face of the crystal 32 tobetested. Cathode 26 of electron discharge device 2'I is heated byfilament heater 33 connected vin parallel with filament heater I5 tosuitable source 34 as shown. Essentially, Fig. 3 is comprised of twounits connected through a common impedance or reactance, the one being agenerator comprising tube I8 and the other a diode vacuum-tube voltmetercomprising tube 2 I, across the input of which the piezo elements 32 areinserted and tested by varying the resonant frequency of the generatorto correspond to any of the resonant frequencies of the piezo-elementunder test. The accuracy of control of frequency is determined by (a)the stability of the oscillator, (b) the calibration, (c) the accuracyof standardization against a known standard frequency, and (d) theaccuracy of the known standard. These four factors may be held to aclose minimum, thereby allowing very fine control 0i frequency.

For purposes of understanding the operation of the system of myinvention illustrated in Fig. 3, the values of the elements employed inone of the practical forms of the circuit of my invention are asfollows:

ltube I8 is as follows:

Variable condenser I 325 micro-microfarads Split stator variablecondenser 2 -Each section 25 micromicrofarads Trimmer condenser 5 ,550rmicro-microfarads Trimmer condenser 'I 5-50 micro-microfarads Fixedcondenser B .70 micro-microfarads Fixed condenser I5 250micro-microfarads Fixed condenser 20 -.0.1 microfarad Fixed condenser 220.1 microfarad Inductance 3 8.5 microhenries 20 turns 1.5" in diameter x1.375" long tapped 1/3 the distance from the grounded end Resistance Il6,000 ohms Resistance 24 10,000 to 30,000 ohms In the arrangement shownin Fig. 3, the frequency is continuously variable from 5,400 kcs./ sec.to 9,200 kos/sec. by varying condenser I and is also variable over akos/sec. range by varying condenser 2 regardless of the setting ofcondenser I. 2a and 2b are the two sections of the dual-section, gangedvariable condenser 2 controlled by one dial or knob, or other control.The range covered by the bandspread control may be Varied by adjustingtrimmer Condensers 5 and 1. Condensers 5 controls the amount of spreadat the lower frequencies while trimmer condenser 'I controls the spreadat the higher frequencies. The control action exerted by both isapproximately equal at mid-frequency.

The capacity of condenser 8 directly affects the range covered or tunedby condenser I, more capacity increasing the range and less capacitydecreasing the range.

The operation of the oscillator embodying the The necessary voltages areappliedrto the cathodes and other elements of the tubes I8 and 2i, andthe movable electrode 3l of the test jig 29 insulated from thesupporting stage 30 by some good low-loss dielectric. The bandspreadcontrol is set at mid-range and a standard crystal 25 i. e., a nishedquartz crystal plate that has been standardized for frequency against aprimary or secondary frequency standard in a suitable holder, isinserted into the standard jacks.

Now condenser I is varied until the 0-1 milliammeter shows maximumdeflection. (incidentally, resistance 24 is chosen so the milliammeteris just deflected fullscale with a good crystal plugged into thestandard jacks 28 and the oscillator tuned to the crystals main orfundamental response.) This process is known as standardizing anoscillator or just standardizing and can be done equally as Well with aprimary or secondary frequency standard and receiver in conjunction withthe oscillator. When the oscillator is standardized the crystal isremoved from the standard jacks and the low-loss insulator from the testjig 29 and a blank unfinished but lapped quartz plate inserted betweenthe movable electrode and the plate. If the main or fundamentalfrequency or response of the blank lies within :i: 50 kos/sec. of thefrequency of the standard crystal it may be determined by rotatingcondenser 2, meanwhile letting condenser l remain as it was whenstandardized, until the milliammeter 23 shows maximum deflection and thereading taken from a precalibrated dial or scale under a pointer carriedby the bandvspread tuning control either added to or subtracted from thefrequency of the standard crystal. The bandspread dial or scale may becalibrated as desired, the equipment in the example hereindescribedbeing calibrated to 50 kos/sec. plus and minus for mid-frequency rangeand 0-100 kcs./sec from right to left in 5 kos/sec. intervals.

By properly standardizing and Calibrating the bandspread range anddialor scale, the overall error may be kept down to 2.5% or less of thebandspread range or .066 of 1% or less of the range covered `bycondenser I constituting the main tuning control in the circuitarrangement illustrated in Fig. 3.

'Ihe above mentioned inherent accuracy of measurement and the fact thatthe bandspread range is constant Within the small amount mentioned aboveat any7 setting of condenser l are the oscillators two most uniquefeatures and coupled with the fact that the crystal is driven or excitedby the radio frequency voltage generated by the oscillator, andappearing at the standard jacks or the electrodes of the "test jig atthe crystals resonant frequency adapts it very veffectively for theproduction ,testing of rough, unnished, lapped blanks to a closetolerance.

Due to the fact that the two sections of the bandspread control 2 ofFig. 2 always exert some control over the resonant frequency of theoircuit, regardless of the setting of the main tuning control l of Fig.2, the ideal control characteristie for each section of the bandspreadcontrol (as illustrated by curves A and B of Fig. 7) is `never realized.Instead curves A and B of Fig.

8 actually obtain for the circuit of Fig. 2, curve C' being theresultant. Consequently some means of compensating for the non-linearityof curve C. Fig. 8, must be employed if accurate work is to be done withthe circuit. 'This is accomplished by employing one or more variablecondensers, or a dual-section or split-stator variable condenser inparallel with one o'rmore sections of the bandspread controh The actionof the compensating condensers is to adjust the minimum and maximumlcapacity of the parallel combination, thereby effectively controllingthe f tuning range of the bandspread control.

The foregoing compensating controls are illustrated in Figs. 4, and 6whereby the ideal resultant curve C of Fig. 7 may be approached verynearly, as shown by the curves C, C and CIV 0f Figs. 9, 10 and 11. InFigs. '7, 8, 9, 10 and 11, the curves A, A', A, A" and AIV represent thecontrol action of the high-frequency control section of the bandspreadcontrol, curves B, B', B", B" and BIV represent the control action ofthe low-frequency control section 2b of the bandspread control, andsince the effects of the two sections 2a and 2b are additive, curves C,C,-C", C'" and CIV represent the resultant action of curves A, B; A',B'; A, B", A, B" and A1", BIV, respectively.

'Ihe curves of Fig. 9 illustrate the control actions and the resultantaction Where a variable condenser 35 is employed as the high frequencycompensating control in the circuit of Fig. 5, all other componentsremaining the same as in Fig. 2.

The curves of Fig. 10 illustrate the control actions and the resultantaction where a variable condenser 36, is employed as the low frequencycompensating control in the circuit of Fig.

`'dial of the fine-tuning control.

4, all other components remaining the same as n Fig. 2.

The curves of Fig. 11 illustrate the control actions and the resultantaction Where a splitstator or dual-section variable condenser 31 isemployed as the compensating control for both high and low frequenciesin the circuit of Fig. 6, all other components remaining the same as inFig. 2.

As may readily be seen from the curves of Figs. 9, 10 and 11, theresultant curves C", C" and CIV are identical with curve C of Fig. 7,but the separate control curves A", A", AIV and B", B'" and BIV onlyapproximate curves A and B of Fig. 7, curves AIV and BIV of Fig. 11 mostnearly approaching the ideal.

The uses to which the above-described control circuits can be put aremanifold. For example, it can be applied to various circuits of the kindshown in Fig. 3, wherein it is used to test lapped quartz blanks foractivity and frequency to a close tolerance before they arehand-finished and put in holders. It can be used in receivers Where itis desired to have a continuous bandspread of frequency; i. e., one thatis constant at any setting of the main tuning control. In conjunctionwith a local piezo-oscillator (frequency to be determined by type ofuse) and a receiver, and used in a stable variable frequency generatorcircuit by standardizing at time of use, it can be used to determinekthe frequency of another signal within a few cycles. This is done byheter- `odyning the signal emitted by the generator against the unknownsignal and detecting the heterodyne beatby audible or electrical orelectromechanical means and then reading frequency by interpolation froma known standard signal (local piezo oscillator) and the calibration o-nthe It can also be used to control the frequency of the emittedradiation of a radio transmitter, allowing the frequency to be variedover a wide range, andjyet, by means of the fine-tuning control andprestandardization against a local piezo-oscillator or other primary orsecondary frequency standard, control the frequency within a few cyclesof the desired frequency.

The applications of this invention, as can be `seen from the foregoing,are almost limitless.

The control circuit can be applied to any device wherein the resonantfrequency is desired to be variable. It can be applied tocapacity-inductance or capacity-resistance or other impedance-tuned`generator or oscillators, to tuned circuits, such as radio frequency,intermediate frequency or audio frequency stages in amplifiers,receivers, lters, etc., or to other tuned circuits wherein anycombination of capacity, inductance, resistance, or

reactance is used.

It will be observed by referring to Fig. 3 of the drawings that theVariable tuned circuit, composed of condensers, l, 2, 5, '1, and 8,inductance 3,

and the tube I8, are so connected that they form an oscillating circuitof the self-excited or electron-coupled type. This circuit is designedto oscillate with or without a crystal in the standard jacks 28 or testjig 29.

A radio frequency voltage is taken from the vself-excited oscillator atan appropriate point on the inductance coil 3 (usually at the pointwhere the cathode ll'of the tube i8 connects to the coil 3), and fedinto a voltage divider which is comprised of the impedance or reactance`24 and the inter-electrode-capacity and input ad- -mittance orresistance of the diode 2l plus the electrode capacity of the crystalwhen it is connected in the circuit either'by inserting it in a holderinto the ystandard jacks 28 or inserting the crystal blank into the testjig 28.

jacks v28 or the test jig 29 and drives the The y'Voltage appearing atthe low end of the impedance 24 is fed to the crystal via the standardcrystal when the frequency generated by the Y oscillator is the same asthe resonant frequency of the crystal. The amplitude of oscillation ofthe crystal is indicated by the meter 23 which is a direct indication ofits quality as an oscillator.

Theoretically, the impedance was designed not only as one leg of thevoltage-divider, but also to isolate the crystal from the tuned circuitof the .self-excited oscillator. Actually, the isolation is not completeand consequently the crystal exerts some control over the resonant'frequency of the system.

The graph in Fig. 12 illustrates this action for a typical crystal,curve 33 representing the action for the crystal in a typical holder andinserted into the standard jacks 28, curve 39 representing the actionfor the same crystal between the electrodes of the test jig 29, vthefrequency difference being due to the fact that the electrodes betweenwhich the crystal is clamped in the holder, and which in turn isinserted into the standard jacks 28, have a predetermined airgap,thereby effectively raising the resonant frequency of the crystal abovewhat it would be if the electrodes were plane or flat as are those ofthe test jig 29. Incidentally, there is a steady deflection of the meter23 of approximately V0.1 ma., due to contact voltage in the diode 2| andthe radio frequency voltage from the divider when the oscillator istuned off resonance of the crystal, or even when there is no crystal inthe circuit.

Now, as in the graph, operating about a central frequency of 5800kes/sec., let us follow the curve 38 withreference to the various pointsindicated and moving in the direction indicated by the arrow. Beginningat the point 40, the amplitude increases very gradually to a point 4l,1,16 of the distance or frequency of point 48 from the limit of curve 38at 43, beyond which point it begins to rise at a more rapid rate untilat a point Q2, 1/6 of the distance 4I from the limit of curve 38 at 43,it rises very rapidly to a maximum at point 43. Any attempt to tune formore amplitude or a lower frequency only results in the crystalamplitude falling almost instantaneously to zero (X1) and the frequencyof the self-excited oscillator jumping to the point X. Now, however, ifthe oscillator is tuned in the reverse direction, higher in frequency,at point Z the frequency will again jump, this time to the point 42, theamplitude of the crystal rising at the same time to this same point. Itis evident, as stated before, that the crystal exercises some controlover the resonant frequency of the selfeexcited oscillator, there beingsome pulling action presy ent between the crystal and the tuned circuit.

Vthereby increasing the rectified direct current 2. Set the bandspreadcontrol 2 at mid-point 'on its dial calibration.

3. Tune or adjust the main tuningv control l until the meter 23indicates a maximum current.

4. Remove the standard crystal 25 from the circuit. Y

5. Do not touch the main tuning control after completion of step 3above.

6. As to any crystal the frequency of which is within the range of thebandspread controls range on either side of the standard crystalsfrequency, its frequency may be determined by rotating the bandspreadcontrol and adding or subtracting from the standard crystal frequency,as the case may be, the amount indicated on the calibration under thepointer on the control knob. Of course the calibration and range of themain tuning and bandspread controls are determined and set according tothe use to which it is to be put.

7j If the instrument has a means of compensation of bandspread rangesuch as condensers 35,

' 35, 3l of Figs. 4, 5 and 6, then this control should be set beforestep #2 to a point determined by the frequency of the standard crystaland the appropriate calibrationA of the dial or scale of the control.

Fig. 13 discloses a, modified form of the circuit of my invention forobtaining greater precision in bandspread control. yIn this arrangementthe parts of the circuit as described in Fig. 6 are employed withtheaddition of variable condensers 44, and 45, interposed in series betweenthe dual control condenser 2 and the dual controlcondenser 31. Thefunction of condensers 44 and `#l5 is to adjust the amount of controlcondenser 3i exerts with respect to dual condenser 2. By independentlyadjusting condensers 44 and 45 a greater precision adjustment of theelectrically tunable circuit is secured.

In Fig. 14 I have shown a further modification of the circuit of myinvention wherein the elements illustrated in Fig. 2 are provided withthe addition of a variable capacity control at 48 in series between therotor of the dual condenser 2 and the impedance device 3. The variablecondenser 48 is provided with a pad or trimmer condenser 4'! in paralleltherewith for increasing the precision adjustment of the circuit.Condenser 41 is set or adjusted so that condenser 46 has that variationin capacity required for compensating for the lack of linearity in dualcondenser 2 as shown in characteristic curve C in Fig.' 8.

In Fig, 15 I have shown a further'modii'lcation of my precisionadjustment circuit in which components are employed similar to thoseillustrated in Fig. 2 except that the trimmer condensers 5 and 1 areconnected together or gange'd as schematically represented at 48 sothata simultaneous capacitychange occurs upon adjustment of condensers E and1 with respect to dual condenser 2. Actually condensers 5 and 'I varythe range of capacity variation of the various sections of the dualcondenser Z and correspondingly control the resonant frequency of thecircuit.

Fig. 16 illustrates a further modified form of the circuit of myinvention in which the condenser constituting the main tuning control Iis ganged to operate simultaneously with condensers and 'l through gangconnection represented at 49 so as to automatically compensate for anyvariation in control exercised by dual condenser 2.

In Fig. 17 I have shown a further modified circuit arrangement for thetuning system of my invention having provision for adjusting the maximumtunable frequency to which the electrical tuning system is responsive. Iprovide a tuning condenser 5| which is variable in operation and isconnected in parallel with the inductance 3 for precisely adjusting theelectrical system to maximum frequency for which the system is tunable.'I'his maximum frequency adjustment is provided in addition to all ofthe other cooperative adjusting means as heretofore explained andparticularly shown in Figs. 2 and 3.

Features of my invention shown but not claimed herein are set forth inmy co-pending divisional applications Serial No. 563,383, for runingsystem, filed `November 14, 1944; Serial No. 553,384, for Electricaltuning system, filed November 14, 1934, and Serial No.563,385, forSelective tuning system, filed November 14, 1944.

While I have described my invention in certain of its preferredembodiments, I realize that modicaticns may be made and no limitationsupon my invention are intended other than may be imposed by the scope ofthe appended claims.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is as follows: An electrical tuning system comprising incombination with an impedance device, a variable tuning condenserhavingr one side thereof connected with a point on said impedancedevice, a dual condenser including a central rotor and a pair ofvariably related capacity areas constituting a pair of condensersections whose capacities vary simultaneously in the same direction,said rotor being electrically connected with another point of saidimpedance device, one of the condenser sections of said pair beingdisposed in series with said variable tuning condenser and the other ofthe condenser sections of said pair being connected in parallel with thesaid variable tuning condenser and in parallel with said impedancedevice.

\ 2. An electrical tuning system comprising in combination with animpedance device, a variable tuning condenser having one side thereofconnected with a point on said impedance device, a dual condenserincluding a central rotor and a pair of variably related'capacity areasconstituting a pair of condenser sections whose capacities varysimultaneously in the same direction, said rotor being electricallyconnecte-d with another point of said impedance device, one of thecondenser sections of said pair being disposed in series with saidvariable tuning condenser and the other of the condenser sections ofsaid pair being connected effectively in parallel with the said variabletuning condenser and effectively in parallel with said impedancedevice,` the capacity of the serially connected section of said dualcondenser having a capacity exerting as much control of the resonantfrequency at maximum capacity setting of said variable tuning condenseras the parallelly connected section of said dual condenser exerts at theminimum capacity setting of said variable condenser whereby said dualcondenser exerts substantially constant control of the frequency at anysetting betweenfand including minimum and maximum capacity of saidvariable tuning condenser.

3. An electrical tuning system comprising in combination withanimpedance device, a variable tuning condenser having one side thereofconnected with a point on said impedance device, a

dual condenser including a central rotor and a pair of variably relatedcapacity areas constituting a pair of condenser sections whosecapacities vary simultaneously in the same direction, said rotor beingelectrically connected with another point of said impedance device, oneof the condenser sections of said pair being disposed in series withsaid variable tuning condenser and the other of the condenser sectionsof said pair being connected eifectively in parallel with the saidvariable tuning condenser and effectively in parallel with saidimpedance device, and independently adjustable variable capacity meansconnected in series with each of the condenser sections of said dualcondenser.

4. An electrical tuning system comprising in combination with animpedance device, a variable tuning condenser having one side thereofconnected with a point on said impedance device,

a dualcondenser including a central rotor and a pair of variably relatedcapacity areas constituting a pair of condenser sections whosecapacities vary simultaneously in the same direction, said rotor beingelectrically connected with another point of said impedance device, oneof the condenser sections of said pair being disposed in series withsaid variable tuning condenser and the other of the condenser sectionsof said pair being effectively connected in parallel with the saidvariable tuning condenser and effectively in parallel with saidimpedance device, independently adjustable variable capacity meansconnected in series with each of the condenser sections of said dualcondenser, and a separate variable condenser connected between saidcentral rotor and a point between one of said independently adjustablevariable condensers and said variable tuning condenser for adjusting theminimum tunable frequency of the electrical tuning system.

5. An electrical tuning system comprising in combination with animpedance device, a variable tuning condenser having one side thereofconnected with a point on said impedance device, a dual condenserincluding a central rotor and a pair of variably related capacity areasconstituting a pair of condenser sections whose capacities varysimultaneously in the same direction, said rotor being electricallyconnected with another point of said impedance device, one of thecondenser sections of said pair being disposed in series with saidvariable tuning condenser and 6. An electrical tuning system comprisingin .Y

combination with an impedance device, a variable tuning condenser havingone side thereof conpoint of said impedance device, one of the con-Vdenser sections of said pair being disposed in series with said variabletuning condenser and the other of .the condenser sections of said pairbeing connected effectively in parallel with the said variable tuningcondenser and effectively in parallel with said impedance device,separate variable capacity means connected in series with each of thecondenser sections of said dual condenser, and means for simultaneouslyadjusting said separate variable capacity means.

7. An electrical tuning system comprising in combination with animpedance device, a variable tuning condenser having one side thereofconnected with a point on said impedance device, a dual condenserincluding a centralrotor and a pair of Variably related capacity areasconstituting a pair ci condenser sections whose capacities varysimultaneously in the same direction, said rotor being electricallyconnected withV another point of said impedance device, one of thecondenser'sections of said pair being disposed in series with saidvariable tuning condenser and the other of the condenser sections ofsaid pair being 'connected effectively in parallel with the saidvariable tuning condenser and eiectively in parallel with said impedancedevice, separate variable capacity means connected in series with eachof the condenser sections of said dual condenser, and means forsimultaneously adjusting the effective capacity of each of'said separatevariable capacity means and said first mentioned variable tuningcondenser.

8. An electrical tuning system comprising in combination with animpedance device, a Variable tuning condenser havin-g one side thereofconnected with a point on said impedance device, a dual condenserincluding a central rotor and a pair of variably related capacity areasconstitutingv a pair of condenser sections Whose capacties varysimultaneously in the same direction, said rotor being electricallyconnected with another point of said impedance device, one of thecondenser sections of said pair vbeing disposed in series with saidvariable tuning con denser and the other of the condenser sections ofsaid pair being effectively connected in parallel with the said variabletuning condenser and eiectively in parallel with said impedance device,independently adjustable variable capacity means connected in serieswith each of the condenser sections of said dual condenser, and aseparate variable condenser connected between said centralrotor and apoint. between one of said independently adjustable variable condensersand said variable tuning condenser, and another variable condenserconnected in parallel with said impedance deviceand with the seriesparallel combination, comprising said variable tuning condenser onesection of said dual condenser and the independently adjustable variablecapacity means connected in series With it, for adjusting saidelectrical tuning system to maximum tunable frequency.

i GEORGE S. WACHTMAN.

